Reflective display based on liquid crystal materials

ABSTRACT

The present invention relates to a high contrast reflective display comprising at least one substrate, at least one electrically conductive layer and at least one close-packed, ordered monolayer of domains of electrically modulated material in a fixed, preferably crosslinked, polymer matrix and a method of making the same.

FIELD OF THE INVENTION

The present invention relates to a high contrast displays.

BACKGROUND OF THE INVENTION

There is significant interest in low cost flexible electronic displays.Typically, such displays comprise a light modulating component embeddedin a binder (most commonly polymer) matrix that is coated over aconductive plastic support. Broadly speaking, a light modulatingcomponent is a material that changes its optical properties such as itsability to reflect or transmit light in response to an electric field.The light modulating component may be a liquid crystalline material suchas a nematic liquid crystal, a chiral nematic or cholesteric liquidcrystal or a ferroelectric liquid crystal. The light modulating materialmay also be a water insoluble liquid containing particles that undergoelectrophoresis or motion such as rotation or translation in response toan electric field. Displays comprising a liquid crystalline material ina polymer matrix are referred to as polymer dispersed liquid crystal(PDLC) displays.

There are two main methods for fabricating PDLC devices: emulsionmethods and phase separation methods. Emulsion methods have beendescribed in U.S. Pat. Nos. 4,435,047 and 5,363,482. The liquid crystalis mixed with an aqueous solution containing polymer. The liquid crystalis insoluble in the continuous phase and an oil-in-water emulsion isformed when the composition is passed through a suitable shearingdevice, such as a homogenizer. The emulsion is coated on a conductivesurface and the water allowed to evaporate. A second conductive surfacemay then be placed on top of the emulsion or imaging layer bylamination, vacuum deposition, or screen printing to form a device.While the emulsion methods are straightforward to implement, dropletsize distributions tend to be broad resulting in a loss in performance.For cholesteric liquid crystal devices, also referred to herein as CLCdevices, this typically means reduced contrast and brightness. Phaseseparation methods were introduced in an effort to overcome thisdifficulty.

Phase separation methods have been outlined in U.S. Pat. No. 4,688,900and in Drzaic, P. S. in Liquid Crystal Dispersions, pgs. 30-51,published by World Scientific, Singapore (1995). The liquid crystal andpolymer, or precursor to the polymer, are dissolved in a common organicsolvent. The composition is then coated on a conductive surface andinduced to phase separate by application of ultraviolet (UV) radiationor by the application of heat or by evaporation of the solvent,resulting in droplets of liquid crystal in a solid polymer matrix. Adevice may then be constructed utilizing this composition. Althoughphase separation methods produce dispersed droplets having more uniformsize distributions, there are numerous problems with this approach. Forexample, the long term photostability of photopolymerized systems is aconcern due to the presence of photoinitiators that produce reactivefree radicals. Photoinitiators not consumed by the polymerizationprocess can continue to produce free radicals that can degrade thepolymer and liquid crystals over time. Furthermore, it is also knownthat UV radiation is harmful to liquid crystals. Specifically, exposureto UV radiation can lead to decomposition of the chiral dopant in acholesteric liquid crystal mixture, resulting in a change in thereflected color. The use of organic solvents may also be objectionablein certain manufacturing environments.

U.S. Pat. No. 6,423,368 and U.S. Pat No. 6,704,073 propose to overcomethe problems associated with the prior art methods through the use ofdroplets of the liquid crystal material prepared using a limitedcoalescence process. In this process, the droplet-water interface isstabilized by particulate species, such as colloidal silica. Surfacestabilization by particulate species such as colloidal silica isparticularly preferred as it can give narrow size distribution and thesize of the droplets can be controlled by the concentration of theparticulate species employed. The materials prepared via this processare also referred to as Pickering Emulsions and are described more fullyby Whitesides and Ross (J. Colloid Interface Sci. 169, 48 (1995)). Theuniform droplets may be combined with a suitable binder and coated on aconductive surface to prepare a device. The process provides improvementin brightness and contrast over prior art processes. It also overcomessome of the problems associated with photoinitators and UV radiation.However, there is still much room for improvement, particularly in termsof the switching voltage or the voltage needed to change the orientationof the liquid crystal from one state to another. The latter has asignificant effect on the overall cost of the display. A low switchingvoltage is extremely desirable for low cost displays.

The device described by U.S. Pat. No. 6,423,368 and U.S. Pat No.6,704,073 suffers from drawbacks because of the structure of the coatedlayer. Undesirably, there may be more than a monolayer of dropletsbetween the two electrodes. Furthermore, the process of coating a heatedemulsion of the liquid crystal in a gelatin binder onto a substrate witha conductive layer and subsequently lowering the temperature of thecoating to change the state of the coated layer from a free flowingliquid to a gel state (referred to as a sol-gel transition) prior todrying the coating results in an extremely uneven distribution ofdroplets of liquid crystal. At the microscopic scale there are regionsof the coating containing overlapping droplets and other regions with nodroplets at all between the electrodes. The uneven distribution ofdroplets results in a decrease in contrast and an increase in switchingvoltage.

U.S. Pat. No. 6,271,898 and U.S. Pat. No. 5,835,174 also describecompositions suitable for flexible display applications that employ veryuniform sized droplets of liquid crystal in a polymer binder. However,no attempt is made to control the thickness or the distribution ofdroplets in the coated layer resulting in less than optimum performance.

U.S. patent application Ser. No. 10/718,900 shows that the maximumcontrast in a bistable chiral nematic liquid crystal display prepared bythe limited coalescence method is obtained when the uniform liquidcrystal domains or droplets are coated as substantially a monolayer onthe first conductive support. The bistable states in these chiralnematic liquid crystal displays are the planar reflecting state and theweakly scattering focal conic state. Back-scattering of light from theweakly scattering focal conic state increases drastically when there ismore than a monolayer of droplets between the conductive surfaces. Whilethe method provides displays with an improvement in brightness andcontrast, it still falls short of optimum performance because thegelatin binder is made to undergo a sol-gel transition prior to dryingof the coating resulting in an uneven structure.

Rudhardt et al. (Applied Physics Letters vol. 82, page 2610, 2003)describe a method of fabricating a light modulating device wherein acomposition containing very uniform droplets of liquid crystal in anaqueous solution of polymer binder is spread on an indium tin oxide(ITO) coated glass surface and the water allowed to evaporate. Thedroplets of liquid crystal spontaneously self-assemble into a hexagonalclose-packed (HCP) monolayer. A second ITO coated glass surface isplaced over the coated layer of droplets as the top electrode tocomplete construction of the device. A uniform monolayer thickness isachieved for the coated layer and the close-packed distribution ofdroplets is also extremely well defined. Both features result in a lowswitching voltage. However, there are numerous problems with thisapproach. Firstly, the uniform droplets of liquid crystal are preparedby extrusion through a thin capillary into a flowing fluid. When adroplet at the tip of the capillary grows to reach critical size,viscous drag exceeds surface tension and breakoff occurs, producinghighly monodisperse emulsions. Clearly, this method of creating onedroplet at a time is not suitable for large scale manufacture. Secondly,the method by which the second (top) electrode is applied may besuitable for construction of small scale displays on rigid substratessuch as glass but is not viable for large area low cost displays onflexible substrates.

US 2003/0137717A1 and US 2004/0217929A1 indicate that a close-packedmonolayer of droplets of the light modulating component may be desirablefor obtaining high brightness and contrast in a polymer dispersedelectrophoretic display. However the method of making droplets describedin these applications is a standard emulsification process that does notresult in emulsions having a narrow size distribution that is desirablefor obtaining close-packed monolayers by spontaneous self-assembly. Thepreferred method of preparing droplets in US 2003/0137717A1 and US2004/0217929A1 also involves encapsulation resulting in droplets orcapsules in the size range of 20 to 200 microns with wall thickness of0.2 to 10 microns. The relatively large droplet size and wall thicknessresult in high switching voltages. The latter is particularly a problemfor bistable CLC devices. Encapsulation is clearly not desirable butthese applications do not teach how a second conducting layer is to beapplied on top of the coated layer of droplets in the absence ofencapsulation. In the absence of encapsulation, droplets of the lightmodulating component may directly come in contact with the organicsolvent in the screen printed conducting ink leading to contamination orpoisoning of the light modulating component. This is particularly aconcern if the light modulating component is a liquid crystal material.

To overcome the difficulties of US 2003/0137717A1 and US 2004/0217929A1,US 2004/0226820A1 teaches that a close-packed monolayer of droplets maybe obtained by using electro-deposition followed by washing after thedroplets have been spread on a suitable surface using a coating knife orcoating head such as a slot die coating head. However, the additionalsteps of electro-deposition and washing are cumbersome and not suitablefor manufacturing on a large scale. Even with these additional steps, aclose-packed monolayer of uniform thickness is not achieved. The rootmean square (RMS) surface roughness is about 6 microns because ofnon-uniform droplets or capsules. This is a very high value of surfaceroughness that would result in irregular or incomplete curing if a UVcurable screen printed conducive ink is used as the second electrode.The irregular curing will result in increased switching voltages.Furthermore, a surface roughness of this magnitude will also result insignificant non-uniformity of switching voltage across the area of thedisplay since the switching voltage is directly related to the thicknessof the coated layer.

US 2003/0137717A1, US 2004/0217929A1 and US 2004/0226820A1 also teachusing polymer latex as the preferred binder. The use of polymer latexmaterials is not desirable for a number of reasons. Many commerciallatex materials contain high boiling organic co-solvents that renderthem unsuitable for use in PDLC films due to the poisoning effect thesolvents have on the liquid crystal or other light modulating component.This is particularly true if the droplets are not encapsulated as isdesirable from the point of view of reduced switching voltage. Latexpolymers also have an affinity for the liquid crystal or other lightmodulating component leading to dissolution of the light modulatingcomponent into the polymer matrix. Furthermore, if the latex is notfully transparent, it can lead to a loss of contrast. Other binderssuggested in US 2004/0217929A1 such as acrylics or polyvinylalcohol aredifficult to fix or cross-link if used alone. Fixing or cross-linking isdesired in order to preserve the close-packed monolayer structure whenother layers are spread over it.

For these reasons, an alternative approach is clearly needed.

PROBLEM TO BE SOLVED

There remains a need for a reduced cost, display having excellentbrightness, high contrast, and low switching voltage.

SUMMARY OF THE INVENTION

The present invention relates to a high contrast reflective displaycomprising at least one substrate, at least one electrically conductivelayer and at least one close-packed, ordered monolayer of domains ofelectrically modulated material in a fixed polymer matrix and a methodof making the same.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention includes several advantages, not all of which areincorporated in a single embodiment. A display according to the presentinvention would be low cost and require low switching voltage. In thecase of a cholesteric or chiral nematic liquid crystal display, theresulting display is expected to have reflectance closer to thetheoretical limit of 50% and higher contrast.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the diffraction pattern caused by Fraunhoferdiffraction of light indicating a close-packed ordered monolayer ofchiral nematic liquid crystal (CLC) droplets (or droplets of the lightmodulating material) in the coating of the invention.

FIG. 2 represents the diffraction pattern by Fraunhofer diffraction oflight indicating a very disordered coating of the CLC droplets,according to the prior art.

FIG. 3 illustrates the electro-optic response of a display device basedon one embodiment of the method of the invention.

FIG. 4 illustrates the electro-optic response of a display device basedon the method of the prior art.

FIG. 5 illustrates the electro-optic response of a display device basedon a second embodiment of the method of the invention.

FIG. 6 illustrates a display based on one embodiment of the invention.

FIG. 7 illustrates a display based on a second embodiment of theinvention.

FIG. 8 illustrates a typical response of a bistable cholesteric orchiral nematic liquid crystal material to voltage pulses.

FIG. 9 illustrates the uniformity of the surface of the coated layer ofthe light modulating component prepared according to the method of theinvention.

FIG. 10 illustrates fixing the architecture of the coated layer of thelight modulating component of uniform thickness according to the methodof the invention.

FIG. 11 illustrates the electro-optic response of a display device usingpolymer latex as binder in the imaging layer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a high contrast reflective displaycomprising at least one substrate, at least one electrically conductivelayer and at least one close-packed, ordered monolayer of domains ofelectrically modulated material in a fixed, preferably crosslinked,polymer matrix and a method of making the same. In the preferredembodiment, the electrically modulated material is a chiral nematicliquid crystal incorporated in a polymer matrix. Chiral nematic liquidcrystalline materials may be used to create electronic displays that areboth bistable and viewable under ambient lighting. Furthermore, theliquid crystalline materials may be dispersed as micron sized dropletsin an aqueous medium, mixed with a suitable binder material and coatedon a flexible conductive support to create potentially low costdisplays. The operation of these displays is dependent on the contrastbetween the planar reflecting state and the weakly scattering focalconic state. In order to derive the maximum contrast from thesedisplays, it is desired that the chiral nematic liquid crystal domainsor droplets are spread on a conductive support as a close-packed orderedmonolayer. It is possible to prepare such an ordered monolayer by firstapplying an aqueous dispersion of chiral nematic liquid crystal domainsto the substrate in the presence of a suitable binder, allowing thedomains or droplets to self-assemble into a close-packed orderedmonolayer, preferably a hexagonal close-packed (HCP) monolayer and thenallowing the binder material to set, become fixed or crosslink topreserve the close-packed ordered monolayer structure so that otheraqueous layers can be spread above the imaging layer without affectingthe close-packed structure.

In general, the light modulating imaging layer contains electricallymodulated material domains dispersed in a binder. For purposes of thepresent invention domains are defined to be synonymous with micellesand/or droplets. The electrically modulated material may beelectrochromic material, rotatable microencapsulated microspheres,liquid crystal materials, cholesteric/chiral nematic liquid crystalmaterials, polymer dispersed liquid crystals (PDLC), polymer stabilizedliquid crystals, surface stabilized liquid crystals, smectic liquidcrystals, ferroelectric material, electroluminescent material or anyother of a very large number of light modulating imaging materials knownin the prior art. The domains of the electrically modulated imaginglayer include droplets having uniform domain size, with few, if any,parasitic domains, which are domains with random or uncontrolled sizesand which have undesirable electro-optical properties, within the driedcoatings, as described in previous patent art.

The display includes a suitable electrically modulated material disposedon a suitable support structure, such as on or between one or moreelectrodes. The term “electrically modulated material” as used herein isintended to include any suitable nonvolatile material. Suitablematerials for the electrically modulated material are described in U.S.patent application Ser. No. 09/393,553 and U.S. Provisional PatentApplication Ser. No. 60/099,888, the contents of both applications areherein incorporated by reference.

The electrically modulated material may also be an arrangement ofparticles or microscopic containers or microcapsules. Each microcapsulecontains an electrophoretic composition of a fluid, such as a dielectricor emulsion fluid, and a suspension of colored or charged particles orcolloidal material. According to one practice, the particles visuallycontrast with the dielectric fluid. According to another example, theelectrically modulated material may include rotatable balls that canrotate to expose a different colored surface area, and which can migratebetween a forward viewing position and/or a rear nonviewing position,such as gyricon. Specifically, gyricon is a material comprised oftwisting rotating elements contained in liquid filled spherical cavitiesand embedded in an elastomer medium. The rotating elements may be madeto exhibit changes in optical properties by the imposition of anexternal electric field. Upon application of an electric field of agiven polarity, one segment of a rotating element rotates toward, and isvisible by an observer of the display. Application of an electric fieldof opposite polarity, causes the element to rotate and expose a second,different segment to the observer. A gyricon display maintains a givenconfiguration until an electric field is actively applied to the displayassembly. Gyricon materials are disclosed in U.S. Pat. No. 6,147,791,U.S. Pat. No. 4,126,854 and U.S. Pat. No. 6,055,091, the contents ofwhich are herein incorporated by reference.

According to one practice, the microcapsules may be filled withelectrically charged white particles in a black or colored dye. Examplesof electrically modulated material suitable for use with the presentinvention are set forth in International Patent Application PublicationNumber WO 98/41899, International Patent Application Publication NumberWO 98/19208, International Patent Application Publication Number WO98/03896, and International Patent Application Publication Number WO98/41898, the contents of which are herein incorporated by reference.

The electrically modulated material may also include material disclosedin U.S. Pat. No. 6,025,896, the contents of which are incorporatedherein by reference. This material comprises charged particles in aliquid dispersion medium encapsulated in a large number ofmicrocapsules. The charged particles can have different types of colorand charge polarity. For example white positively charged particles canbe employed along with black negatively charged particles. The describedmicrocapsules are disposed between a pair of electrodes, such that adesired image is formed and displayed by the material by varying thedispersion state of the charged particles. The dispersion state of thecharged particles is varied through a controlled electric field appliedto the electrically modulated material.

Further, the electrically modulated material may include a thermochromicmaterial. A thermochromic material is capable of changing its statealternately between transparent and opaque upon the application of heat.In this manner, a thermochromic imaging material develops images throughthe application of heat at specific pixel locations in order to form animage. The thermochromic imaging material retains a particular imageuntil heat is again applied to the material. Since the rewritablematerial is transparent, UV fluorescent printings, designs and patternsunderneath can be seen through.

The electrically modulated material may also include surface stabilizedferrroelectric liquid crystals (SSFLC). Surface stabilized ferroelectricliquid crystals confining ferroelectric liquid crystal material betweenclosely spaced glass plates to suppress the natural helix configurationof the crystals. The cells switch rapidly between two opticallydistinct, stable states simply by alternating the sign of an appliedelectric field.

Magnetic particles suspended in an emulsion comprise an additionalimaging material suitable for use with the present invention.Application of a magnetic force alters pixels formed with the magneticparticles in order to create, update or change human and/or machinereadable indicia. Those skilled in the art will recognize that a varietyof bistable nonvolatile imaging materials are available and may beimplemented in the present invention.

The electrically modulated material may also be configured as a singlecolor, such as black, white or clear, and may be fluorescent,iridescent, bioluminescent, incandescent, ultraviolet, infrared, or mayinclude a wavelength specific radiation absorbing or emitting material.There may be multiple layers of electrically modulated material.Different layers or regions of the electrically modulated material mayhave different properties or colors. Moreover, the characteristics ofthe various layers may be different from each other. For example, onelayer can be used to view or display information in the visible lightrange, while a second layer responds to or emits ultraviolet light. Thenonvisible layers may alternatively be constructed of nonelectricallymodulated material based materials that have the previously listedradiation absorbing or emitting characteristics. The electricallymodulated material employed in connection with the present inventionpreferably has the characteristic that it does not require power tomaintain display of indicia.

The most preferred electrically modulated material is a light modulatingmaterial, such as a liquid crystalline material. The liquid crystallinematerial can be one of many different liquid crystal phases such as;nematic (N), chiral nematic (N*), or smectic, depending upon thearrangement of the molecules in the mesophase. Chiral nematic liquidcrystal (N*LC) displays are preferably reflective, that is, no backlightis needed, and can function without the use of polarizing films or acolor filter.

Chiral nematic liquid crystal refers to the type of liquid crystalhaving finer pitch than that of twisted nematic and super twistednematic used in commonly encountered liquid crystal devices. Chiralnematic liquid crystals are so named because such liquid crystalformulations are commonly obtained by adding chiral agents to hostnematic liquid crystals. Chiral nematic liquid crystals may be used toproduce bistable or multi-stable displays. These devices havesignificantly reduced power consumption due to their nonvolatile“memory” characteristic. Since such displays do not require a continuousdriving circuit to maintain an image, they consume significantly reducedpower. Chiral nematic displays are bistable in the absence of a field,the two stable textures are the reflective planar texture and the weaklyscattering focal conic texture. In the planar texture, the helical axesof the chiral nematic liquid crystal molecules are substantiallyperpendicular to the substrate upon which the liquid crystal isdisposed. In the focal conic state the helical axes of the liquidcrystal molecules are generally randomly oriented. Adjusting theconcentration of chiral dopants in the chiral nematic material modulatesthe pitch length of the mesophase and, thus, the wavelength of radiationreflected. Chiral nematic materials that reflect infrared radiation andultraviolet have been used for purposes of scientific study. Commercialdisplays are most often fabricated from chiral nematic materials thatreflect visible light. Some known LCD devices include chemically etched,transparent, conductive layers overlying a glass substrate as describedin U.S. Pat. No. 5,667,853, incorporated herein by reference. Suitablechiral nematic liquid crystal compositions preferably have a positivedielectric anisotropy and include chiral material in an amount effectiveto form focal conic and twisted planar textures. Chiral nematic liquidcrystal materials are preferred because of their excellent reflectivecharacteristics, bistability and gray scale memory.

Modern chiral nematic liquid crystal materials usually include at leastone nematic host combined with a chiral dopant. In general, the nematicliquid crystal phase is composed of one or more mesogenic componentscombined to provide useful composite properties. The nematic componentof the chiral nematic liquid crystal mixture may be comprised of anysuitable nematic liquid crystal mixture or composition havingappropriate liquid crystal characteristics. Nematic liquid crystalssuitable for use in the present invention are preferably composed ofcompounds of low molecular weight selected from nematic or nematogenicsubstances, for example from the known classes of the azoxybenzenes,benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexylbenzoates, phenyl or cyclohexyl esters of cyclohexanecarboxylic acid,phenyl or cyclohexyl esters of cyclohexylbenzoic acid, phenyl orcyclohexyl esters of cyclohexylcyclohexanecarboxylic acid,cyclohexylphenyl esters of benzoic acid, of cyclohexanecarboxyiic acidand of cyclohexylcyclohexanecarboxylic acid, phenyl cyclohexanes,cyclohexyibiphenyls, phenyl cyclohexylcyclohexanes,cyclohexylcyclohexanes, cyclohexylcyclohexenes,cyclohexylcyclohexylcyclohexenes, 1,4-bis-cyclohexylbenzenes,4,4-bis-cyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl-or cyclohexylpyridines, phenyl- or cyclohexylpyridazines, phenyl- orcyclohexyidioxanes, phenyl- or cyclohexyl-1,3-dithianes,1,2-diphenylethanes, 1,2-dicyclohexylethanes,1-phenyl-2-cyclohexylethanes,1-cyclohexyl-2-(4-phenylcyclohexyl)ethanes,1-cyclohexyl-2′,2-biphenylethanes, 1-phenyl-2-cyclohexylphenylethanes,optionally halogenated stilbenes, benzyl phenyl ethers, tolanes,substituted cinnamic acids and esters, and further classes of nematic ornematogenic substances. The 1,4-phenylene groups in these compounds mayalso be laterally mono- or difluorinated. The liquid crystallinematerial of this preferred embodiment is based on the achiral compoundsof this type. The most important compounds, that are possible ascomponents of these liquid crystalline materials, can be characterizedby the following formula R′—X—Y-Z-R″ wherein X and Z, which may beidentical or different, are in each case, independently from oneanother, a bivalent radical from the group formed by -Phe-, -Cyc-,-Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, —B-Phe- and —B-Cyc-,wherein Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cycis trans-1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr ispyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl,and B is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl,pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl. Y in these compounds isselected from the following bivalent groups —CH═CH—, —C≡C—, —N═N(O)—,—CH═CY′—, —CH═N(O)—, —CH2-CH2-, —CO—O—, —CH2-O—, —CO—S—, —CH2-S—,—COO-Phe-COO— or a single bond, with Y′ being halogen, preferablychlorine, or —CN, R′ and R″ are, in each case, independently of oneanother, alkyl, alkenyl, alkoxy, alkenyloxy, alkanoyloxy, alkoxycarbonylor alkoxycarbonyloxy with 1 to 18, preferably 1 to 12 C atoms, oralternatively one of R′ and R″ is —F, —CF3, —OCF3, —Cl, —NCS or —CN. Inmost of these compounds R′ and R′ are, in each case, independently ofeach another, alkyl, alkenyl or alkoxy with different chain length,wherein the sum of C atoms in nematic media generally is between 2 and9, preferably between 2 and 7. The nematic liquid crystal phasestypically consist of 2 to 20, preferably 2 to 15 components. The abovelist of materials is not intended to be exhaustive or limiting. Thelists disclose a variety of representative materials suitable for use ormixtures, which comprise the active element in light modulating liquidcrystal compositions. Chiral nematic liquid crystal materials and cells,as well as polymer stabilized chiral nematic liquid crystals and cells,are well known in the art and described in, for example, U.S. patentapplication Ser. No. 07/969,093, Ser. No. 08/057,662, Yang et al., Appl.Phys. Lett. 60(25) pp 3102-04 (1992), Yang et al., J. Appl. Phys. 76(2)pp 1331 (1994), published International Patent Application No.PCT/US92/09367, and published International Patent Application No.PCT/US92/03504, all of which are incorporated herein by reference.

Suitable commercial nematic liquid crystals include, for example, E7,E44, E48, E31, E80, BL087, BL101, ZLI-3308, ZLI-3273, ZLI-5048-000,ZLI-5049-100, ZLI-5100-100, ZLI-5800-000, MLC-6041-100.TL202, TL203,TL204 and TL205 manufactured by E. Merck (Darmstadt, Germany). Althoughnematic liquid crystals having positive dielectric anisotropy, andespecially cyanobiphenyls, are preferred, virtually any nematic liquidcrystal known in the art, including those having negative dielectricanisotropy should be suitable for use in the invention. Other nematicmaterials may also be suitable for use in the present invention as wouldbe appreciated by those skilled in the art.

The chiral dopant added to the nematic mixture to induce the helicaltwisting of the mesophase, thereby allowing reflection of visible light,can be of any useful structural class. The choice of dopant depends uponseveral characteristics including among others its chemicalcompatibility with the nematic host, helical twisting power, temperaturesensitivity, and light fastness. Many chiral dopant classes are known inthe art: for example, G. Gottarelli and G. Spada, Mol. Cryst. Liq.Crys., 123, 377 (1985), G. Spada and G. Proni, Enantiomer, 3, 301 (1998)and references therein. Typical well known dopant classes include1,1-binaphthol derivatives, isosorbide and similar isomannide esters asdisclosed in U.S. Pat. No. 6,217,792, TADDOL derivatives as disclosed inU.S. Pat. No. 6,099,751, and the pending spiroindanes esters asdisclosed in U.S. patent application Ser. No. 10/651,692 by T. Welter etal., filed Aug. 29, 2003, titled “Chiral Compounds And CompositionsContaining The Same,” hereby incorporated by reference.

The pitch length of the liquid crystal materials may be adjusted basedupon the following equation (1):λ_(nm)=n_(av)p₀where λ_(max) is the peak reflection wavelength, that is, the wavelengthat which reflectance is a maximum, n_(av) is the average index ofrefraction of the liquid crystal material, and p₀ is the natural pitchlength of the chiral nematic helix. Definitions of chiral nematic helixand pitch length and methods of its measurement, are known to thoseskilled in the art such as can be found in the book, Blinov, L. M.,Electro-optical and Magneto-Optical Properties of Liquid Crystals, JohnWiley & Sons Ltd. 1983. The pitch length is modified by adjusting theconcentration of the chiral material in the liquid crystal material. Formost concentrations of chiral dopants, the pitch length induced by thedopant is inversely proportional to the concentration of the dopant. Theproportionality constant is given by the following equation (2):p ₀=1/(HTP.c)where c is the concentration of the chiral dopant and HTP is theproportionality constant.

For some applications, it is desired to have liquid crystal mixturesthat exhibit a strong helical twist and thereby a short pitch length.For example in liquid crystalline mixtures that are used in selectivelyreflecting chiral nematic displays, the pitch has to be selected suchthat the maximum of the wavelength reflected by the chiral nematic helixis in the range of visible light. Other possible applications arepolymer films with a chiral liquid crystalline phase for opticalelements, such as chiral nematic broadband polarizers, filter arrays, orchiral liquid crystalline retardation films. Among these are active andpassive optical elements or color filters and liquid crystal displays,for example STN, TN, AMD-TN, temperature compensation, polymer free orpolymer stabilized chiral nematic texture (PFCT, PSCT) displays.Possible display industry applications include ultralight, flexible, andinexpensive displays for notebook and desktop computers, instrumentpanels, video game machines, videophones, mobile phones, hand held PCs,PDAs, e-books, camcorders, satellite navigation systems, store andsupermarket pricing systems, highway signs, informational displays,smart cards, toys, and other electronic devices.

The liquid crystalline droplets or domains are typically dispersed in acontinuous binder. In one embodiment, a chiral nematic liquid crystalcomposition may be dispersed in a continuous polymeric matrix. Suchmaterials are referred to as “polymer dispersed liquid crystal”materials or “PDLC” materials. Suitable hydrophilic binders include bothnaturally occurring substances such as proteins, protein derivatives,cellulose derivatives (for example cellulose esters), gelatins andgelatin derivatives, polysaccaharides, casein, and the like, andsynthetic water permeable colloids such as poly(vinyl lactams),acrylamide polymers, latex, poly(vinyl alcohol) and its derivatives,hydrolyzed polyvinyl acetates, polymers of alkyl and sulfoalkylacrylates and methacrylates, polyamides, polyvinyl pyridine, acrylicacid polymers, maleic anhydride copolymers, polyalkylene oxide,methacrylamide copolymers, polyvinyl oxazolidinones, maleic acidcopolymers, vinyl amine copolymers, methacrylic acid copolymers,acryloyloxyalkyl acrylate and methacrylates, vinyl imidazole copolymers,vinyl sulfide copolymers, and homopolymer or copolymers containingstyrene sulfonic acid. Gelatin is preferred.

Gelatin is derived from a material called collagen. Collagen has a highcontent of glycine and of the imino acids proline and hydroxyproline. Ithas a triple helix structure made up of three parallel chains. Whencollagen in water is heated above a certain temperature, it willdenature to form gelatin. Concentrated gelatin solutions form rigid gelswhen cooled. This phenomenon is known as sol-gel transition or thermalgelation and is the result of secondary bonding, such as hydrogenbonding, between gelatin molecules in solution. It should be noted thatthis property is not limited to gelatin; for example, aqueous solutionsof agar, a polysaccharide from seaweed, will also form rigid gels uponcooling. Partial renaturation of gelatin also occurs upon cooling. Thelatter refers to the formation of triple helix collagen-like structures.The structures do not form if gelatin is not chill set prior to drying.In other words, molecules of gelatin remain in a random coilconfiguration if the coating is dried at a temperature that is above thesol-gel transition temperature. The presence of helix structures may bedetected by x-ray diffraction. Chill set gelatin containing molecules ina helix configuration has relatively low solubility in cold water andorganic solvents compared to the random coil gelatin. This propertyenables chill set gelatin to be an effective barrier between the organicsolvent in printed conductive inks and the light modulating material.

Useful “gelatins,” as that term is used generically herein, includealkali treated gelatin (cattle bone or hide gelatin), acid treatedgelatin (pigskin gelatin), fish skin gelatin and gelatin derivativessuch as acetylated gelatin, and phthalated gelatin. Any type of gelatinmay be used, provided the gelatin has sufficient molecular weight toallow the crosslinker to crosslink or the fixative to fix or set. Fishgelatins have lower imino acid content compared to mammalian gelatins.The sol-gel transition temperature or thermal gelation temperature orchill set temperature is lower if the imino acid content is smaller. Forexample, the chill set temperature of gelatins derived from deep waterfish such as cod, haddock or pollock is significantly lower than that ofcattle gelatin. Aqueous solutions of these gelatins remain fluid untilabout 10° C. whereas solutions of cattle gelatin will gel at roomtemperature. Other hydrophilic colloids that can be utilized alone or incombination with gelatin include dextran, gum arabic, zein, casein,pectin, collagen derivatives, collodion, agar-agar, arrowroot, albumin,and the like. Still other useful hydrophilic colloids are water solublepolyvinyl compounds such as polyvinyl alcohol, polyacrylamide,poly(vinylpyrrolidone), and the like. Useful liquid crystal to gelatinratios should be between 6:1 and 0.5:1 liquid crystal to gelatin,preferably 3:1.

Other organic binders such as polyvinyl alcohol (PVA) or polyethyleneoxide (PEO) can be used as minor components of the binder in addition togelatin. Such compounds are preferably machine coatable on equipmentassociated with photographic films.

It is desirable that the binder has a low ionic content. The presence ofions in such a binder hinders the development of an electrical fieldacross the dispersed liquid crystal material. Additionally, ions in thebinder can migrate in the presence of an electrical field, chemicallydamaging the light modulating layer. The coating thickness, size of theliquid crystal domains, and concentration of the domains of liquidcrystal materials are designed for optimum optical properties.Heretofore, the dispersion of liquid crystals is performed using shearmills or other mechanical separating means to form domains of liquidcrystal within light modulating layer.

A conventional surfactant can be added to the emulsion to improvecoating of the layer. Surfactants can be of conventional design, and areprovided at a concentration that corresponds to the critical micelleconcentration (CMC) of the solution. A preferred surfactant is AerosolOT, commercially available from Cytec Industries, Inc.

In a preferred embodiment, the liquid crystal and gelatin emulsion arecoated and dried to optimize the optical properties of the lightmodulating layer. In one embodiment, the layer is coated to provide afinal coating containing a substantial monolayer of N*LC domains. Theterm “substantial monolayer” is defined by the Applicants to mean that,in a direction perpendicular to the plane of the display, there is nomore than a single layer of domains sandwiched between the electrodesover 90% of the area of the display (or the imaging layer).

The amount of material needed for a monolayer can be determined bycalculation based on individual domain size. Furthermore, improvedviewing angle and broadband features may be obtained by appropriatechoice of differently doped domains based on the geometry of the coateddroplet and the Bragg reflection condition.

The addition of a bacteriostat prevents gelatin degradation duringemulsion storage and during material operation. The gelatinconcentration in the emulsion when coated is preferably between about 2and 20 weight percent based on the weight of the emulsion. In the finalemulsion, the liquid crystal material may be dispersed at 15%concentration in a 5% gelatin aqueous solution.

A crosslinking agent or hardener may be used to preserve thearchitecture of the close-packed monolayer of coated droplets after ithas been formed by self-assembly. Other methods of fixing thearchitecture of the close-packed monolayer of domains may also be used,although crosslinking is preferred. The effects of the crosslinker maybe characterized based on the reaction of certain amino acid residues ingelatin. For example, the amount of histadine is typically reduced fromabout 4 residues per 1000 to less than 2.5 residues per 1000 uponcross-linking. Also the amount of hydroxylysine is reduced from about6.9 residues per 1000 to less than 5.1 residues per 1000. Manyconventional hardeners are known to crosslink gelatin. Gelatincrosslinking agents (i.e., the hardener) are included in an amount of atleast about 0.01 wt. % and preferably from about 0.1 to about 10 wt. %based on the weight of the solid dried gelatin material used (by driedgelatin it is meant substantially dry gelatin at ambient conditions asfor example obtained from Eastman Gel Co., as compared to swollengelatin), and more preferably in the amount of from about 1 to about 5percent by weight. More than one gelatin crosslinking agent can be usedif desired. Suitable hardeners may include inorganic, organic hardeners,such as aldehyde hardeners and olefinic hardeners. Inorganic hardenersinclude compounds such as aluminum salts, especially the sulfate,potassium and ammonium alums, ammonium zirconium carbonate, chromiumsalts such as chromium sulfate and chromium alum, and salts of titaniumdioxide, and zirconium dioxide. Representative organic hardeners orgelatin crosslinking agents may include aldehyde and related compounds,pyridiniums, olefins, carbodiimides, and epoxides. Thus, suitablealdehyde hardeners include formaldehyde and compounds that contain twoor more aldehyde functional groups such as glyoxal, gluteraldehyde andthe like. Other preferred hardeners include compounds that containblocked aldehyde functional groups such as aldehydes of the typetetrahydro-4-hydroxy-5-methyl-2(1H)-pyrimidinone polymers (Sequa SUNREZ®700), polymers of the type having a glyoxal polyol reaction productconsisting of 1 anhydroglucose unit: 2 glyoxal units (SEQUAREZ® 755obtained from Sequa Chemicals, Inc.), DME-Melamine non-formaldehyderesins such as Sequa CPD3046-76 obtained from Sequa Chemicals Inc., and2,3-dihydroxy-1,4-dioxane (DHD). Thus, hardeners that contain activeolefinic functional groups include, for example,bis-(vinylsulfonyl)-methane (BVSM), bis-(vinylsulfonyl-methyl) ether(BVSME), 1,3,5-triacryloylhexahydro-s-triazine, and the like. In thecontext of the present invention, active olefinic compounds are definedas compounds having two or more olefinic bonds, especially unsubstitutedvinyl groups, activated by adjacent electron withdrawing groups (TheTheory of the Photographic Process, 4th Edition, T. H. James, 1977,Macmillan Publishing Co., page 82). Other examples of hardening agentscan be found in standard references such as The Theory of thePhotographic Process, T. H. James, Macmillan Publishing Co., Inc. (NewYork 1977) or in Research Disclosure, September 1996, Vol. 389, Part IIB(Hardeners) or in Research Disclosure, September 1994, Vol. 365, Item36544, Part IIB (Hardeners). Research Disclosure is published by KennethMason Publications, Ltd., Dudley House, 12 North St., Emsworth,Hampshire P010 7DQ, England. Olefinic hardeners are most preferred, asdisclosed in U.S. Pat. Nos. 3,689,274, 2,994,611, 3,642,486, 3,490,911,3,635,718, 3,640,720, 2,992,109, 3,232,763, and 3,360,372.

Among hardeners of the active olefin type, a preferred class ofhardeners particularly are compounds comprising two or more vinylsulfonyl groups. These compounds are hereinafter referred to as “vinylsulfones”. Compounds of this type are described in numerous patentsincluding, for example, U.S. Pat. Nos. 3,490,911, 3,642,486, 3,841,872and 4,171,976. Vinyl sulfone hardeners are believed to be effective ashardeners as a result of their ability to crosslink polymers making upthe colloid.

The liquid crystalline droplets or domains may be formed by any method,known to those of skill in the art, which will allow control of thedomain size. For example, Doane et al. (Applied Physics Letters, 48, 269(1986)) disclose a PDLC comprising approximately 0.4 μm droplets ofnematic liquid crystal 5CB in a polymer binder. A phase separationmethod is used for preparing the PDLC. A solution containing monomer andliquid crystal is filled in a display cell and the material is thenpolymerized. Upon polymerization the liquid crystal becomes immiscibleand nucleates to form droplets. West et al. (Applied Physics Letters 63,1471 (1993)) disclose a PDLC comprising a chiral nematic mixture in apolymer binder. Once again a phase separation method is used forpreparing the PDLC. The liquid crystal material and polymer (a hydroxyfunctionalized polymethylmethacrylate) along with a crosslinker for thepolymer are dissolved in a common organic solvent toluene and coated onan indium tin oxide (ITO) substrate. A dispersion of the liquid crystalmaterial in the polymer binder is formed upon evaporation of toluene athigh temperature. The phase separation methods of Doane et al. and Westet al. require the use of organic solvents that may be objectionable incertain manufacturing environments.

In a preferred embodiment, a method referred to as “limited coalescence”is used to form uniformly sized emulsions of liquid crystallinematerial. For example, the liquid crystal material can be homogenized inthe presence of finely divided silica, a coalescence limiting material,such as LUDOX® from DuPont Corporation. A promoter material can be addedto the aqueous bath to drive the colloidal particles to theliquid-liquid interface. In a preferred embodiment, a copolymer ofadipic acid and 2-(methylamino)ethanol can be used as the promotingagent in the water bath. The liquid crystal material can be dispersedusing ultrasound to create liquid crystal domains below 1 micron insize. When the ultrasound energy is removed, the liquid crystal materialcoalesces into domains of uniform size. The limited coalescence processis described more fully by Whitesides and Ross (J. Colloid InterfaceSci. 169, 48 (1995)), by Giermanska-Kahn, Schmitt, Binks andLeal-Calderon (Langmuir, 18, 2515 (2002)), and U.S. Pat. No. 6,556,262,all incorporated herein by reference.

The distribution of droplet sizes is such that the coefficient ofvariation (cv) defined as the standard deviation of the distributiondivided by the arithmetic mean is less than 0.25, preferably less than0.2 and most preferably less than 0.15. The limited coalescent materialscan be coated using a photographic emulsion coating machine onto sheetsof polyester having an ITO coating with a sheet conductivity of 300 ohmsper square. The coating can be dried to provide a polymericallydispersed cholesteric coating. By using limited coalescence, there arefew, if any, parasitic smaller domains (having undesirableelectro-optical properties) within the dried coatings.

The size ranges of domains in the dried coating are varied as themixture dries and the domains flatten. In one embodiment, the resultingdomains are flattened by the drying process and have on average athickness substantially less than their length. The flattening of thedomains can be achieved by proper formulation and sufficiently rapiddrying of the coating.

Preferably, the domains are flattened spheres and have on average athickness substantially less than their length, preferably at least 50%less. More preferably, the domains on average have a thickness (depth)to length ratio of 1:2 to 1:6. The flattening of the domains can beachieved by proper formulation and sufficiently rapid drying of thecoating. The domains preferably have an average diameter of 2 to 30microns. The imaging layer preferably has a thickness of 10 to 150microns when first coated and 2 to 20 microns when dried. Mostpreferably the imaging layer or light modulating layer has a thicknessbetween 2 to 6 microns, particularly if the light modulating material isa chiral nematic liquid crystal.

The flattened domains of liquid crystal material can be defined ashaving a major axis and a minor axis. In a preferred embodiment of adisplay or display sheet, the major axis is larger in size than the cell(or imaging layer) thickness for a majority of the domains. Such adimensional relationship is shown in U.S. Pat. No. 6,061,107, herebyincorporated by reference in its entirety.

In U.S. Pat. No. 3,600,060, incorporated herein by reference, thedomains of the dried light modulating material had particle size varyingin diameter by a ratio of 10:1. This creates large domains and smallerparasitic domains. Parasitic domains have reduced characteristics whencompared with optimized larger domains. The reduced characteristicsinclude reduced brightness and if the parasitic domains are small enoughdiminished bistability of the cholesteric liquid crystal.

The dispersed domains have an average diameter of 2 to 30 microns,preferably 5 to 15 microns. The domains are dispersed in an aqueoussuspension. The size ranges for the dried coating are varied as themixture dries and the domains flatten.

By varying the amount of silica and copolymer relative to the liquidcrystalline material, uniform domain size emulsions of the desiredaverage diameter (by microscopy), can be produced. This process producesdomains of a selected average diameter.

The resulting domains are flattened by the drying process and have onaverage a thickness substantially less than their length, preferably atleast 50% less. More preferably, the domains on average have a thickness(depth) to length ratio of 1:2 to 1:10.

For optimal performance one requires a monolayer of coated dropletshaving a close-packed structure of uniform thickness. Calculations byYang and Mi (J. Phys. D: Appl. Phys. Vol. 33, page 672, 2000) have shownthat for a chiral nematic liquid crystalline material of a givenhandedness, maximum reflectance is obtained if the thickness of thechiral nematic liquid crystal material between the electrodes is aboutten times the pitch of the chiral nematic helix. For a green reflectingchiral nematic liquid crystal material with λ_(max) of 550 nm and n_(av)of 1.6 the pitch is 344 nm. Therefore, maximum reflectance is obtainedfor a 3.4 μm thick layer of this material. For chiral nematic liquidcrystal materials that reflect in the red and near infrared portions ofthe spectrum, the pitch and therefore the thickness of the coated layerneeded for maximum reflectance will be somewhat higher but even in thesecases a thickness of about 5 μm is sufficient, if the refractive indexis close to 1.6. In other words, increasing the thickness of the layerbeyond this does not provide an increase in reflectance.

It is also well documented that the switching voltage increases linearlywith thickness. Since it is desirable to have the lowest possibleswitching voltage, a uniform thickness of about 5 μm is most preferredfor the coated layer of droplets, provided the droplets have aclose-packed structure. Under certain conditions, monodisperse dropletsof the light modulating material will spontaneously self-assemble on asurface into a hexagonal close-packed (HCP) structure. The process hasbeen described in detail by Denkov et al. (Nature, vol. 361, p. 26,1993). When an aqueous suspension of droplets is spread on a surface,the droplets initially assume a random, disordered or uncorrelateddistribution. However, as a function of drying, when the level of waterreaches the top of the droplets, there is a strong attractive forceknown as the capillary force that drives the droplets into aclose-packed ordered or correlated structure. The attractive energy ofthe capillary force is much greater than the thermal energy. However, itis important that lateral movement of droplets is not impeded by astrong attraction to the surface or by an increase in viscosity of themedium in which they are suspended. The latter would happen if thebinder is gelatin and the coated layer of droplets is chill set prior todrying.

The formation of a close-packed structure in two dimensions, startingfrom a random distribution of droplets, is sometimes referred to astwo-dimensional crystallization and should have a monodispersepopulation of droplets or a population of droplets having lowpolydispersity (Kumacheva et al. Physical Review Letters vol. 91, page1283010-1, 2003). A population of droplets of light modulating materialhaving sufficiently low polydispersity to create a close-packedstructure may be achieved by the limited coalescence process. Theclose-packed structure is readily observable under an opticalmicroscope. Furthermore, the close-packed structure has a repeat patternor periodicity wherein the repeat distance is of the order of thewavelength of visible light. A coating having such a pattern exhibitsFraunhofer diffraction when placed before a source of visible light suchas a visible light laser. The phenomenon of Fraunhofer diffraction isdescribed more fully by Lisensky et al. Journal of Chemical Education,vol. 68, February 1991.

For perfectly monodisperse droplets (cv less than 0.1), a hexagonalclose-packed (HCP) structure is obtained. The diffraction pattern forsuch a structure is in the form of spots. If there is a minor level ofpolydispersity (cv between 0.1 and 0.2), the diffraction pattern of theclose-packed structure is in the form of a single ring or a set ofconcentric rings.

The close-packed monolayer structure of coated droplets may be preservedby fixing or crosslinking the binder. This allows a second aqueous layerto be coated above the layer containing the light modulating materialwithout disturbing the close-packed organization. In a preferredembodiment, the second layer functions as a protective overcoat for thelight modulating material.

A preferred embodiment of the display device shown in FIG. 6 comprises aclear flexible support 101 with a clear conducting layer 102. Theimaging layer or light modulating layer (layer 1) contains aclose-packed monolayer of droplets of the light modulating material 104along with crosslinked binder 103 in a random coil configuration. Aprotective overcoat 105 is composed of polymer in a helix configuration.The second electrode 106 comprises screen printed carbon conductive ink.

A second preferred embodiment of the display device, shown in FIG. 7,comprises a clear flexible support 101 with a clear conducting layer102. The imaging layer or light modulating layer (layer 1) contains aclose-packed monolayer of droplets of the light modulating material 104along with crosslinked binder 103 in a random coil configuration. Aprotective overcoat 107 is composed of polymer in a helix configurationand dispersed carbon black for improved contrast. The second electrode108 comprises screen printed silver conductive ink.

In addition to binder and hardener, liquid crystal layers may alsocontain a small amount of light absorbing colorant, preferably anabsorber dye. It is preferred that an absorbing dye is used toselectively absorb back scattered light from the focal conic state atthe lowest wavelengths in the visible part of the spectrum. Further, thecolorant selectively absorbs similarly scattered light from the planarstate, while only minimally absorbing the main body of reflected light.The colorants may include both dyes and pigments. The colorant mayabsorb light components, which may cause turbidity of color in the colordisplay performed by selective reflection of the liquid crystal or maycause lowering of a transparency in the transparent state of the liquidcrystal, and therefore can improve the display quality. Two or more ofthe components in the liquid crystal display may contain a coloringagent. For example, both the polymer and the liquid crystal may containthe coloring agent. Preferably, a colorant is selected, which absorbsrays in a range of shorter wavelengths than the selective reflectionwavelength of the liquid crystal.

Any amount of colorant may be used, provided that addition of thecolorant does not remarkably impair the switching characteristics of theliquid crystal material for display. In addition, if the polymericbinder is formed by polymerization, the addition does not inhibit thepolymerization. An exemplary amount of colorant is from at least 0.1weight % to 5 weight % of the liquid crystal material.

In a preferred embodiment, the colorants, preferably absorber dyes, areincorporated directly in the chiral nematic liquid crystal materials.Any colorants that are miscible with the cholesteric liquid crystalmaterials are useful for this purpose. Most preferred are colorants thatare readily soluble in toluene. By readily soluble is meant a solubilitygreater than 1 gram per liter, more preferably greater than 10 grams perliter and most preferably greater than 100 grams per liter. Toluenesoluble dyes most compatible with the cholesteric liquid crystalmaterials are anthraquinone dyes such as Sandoplast Blue 2B fromClariant Corporation, phthalocyanine dyes such as Savinyl Blue GLS fromClariant Corporation or Neozapon Blue 807 from BASF Corporation, methinedyes such as Sandoplast Yellow 3G from Clariant Corporation or metalcomplex dyes such as Neozapon Yellow 157, Neozapon Orange 251, NeozaponGreen 975, Neozapon Blue 807 or Neozapon Red 365 from BASF Corporation.Other colorants are Neopen Blue 808, Neopen Yellow 075, Sudan Orange 220or Sudan Blue 670 from BASF Corporation. Other types of colorants mayinclude various kinds of dyestuff such as dyestuff for resin coloringand dichromatic dyestuff for liquid crystal display. The dyestuff forresin coloring may be SPR RED 1 (manufactured by Mitsui Toatsu SenryoCo., Ltd.). The dichromatic dyestuff for liquid crystal is specificallySI-424 or M-483 (both manufactured by Mitsui Toatsu Senryo Co., Ltd.).

Another aspect of the present invention relates to a display sheetcomprising a substrate, an electrically conductive layer formed over thesubstrate, and a liquid crystal containing imaging layer comprising achiral nematic material formed by the above described methods disposedover the electrically conductive layer.

As used herein, the phrase a “liquid crystal display” (LCD) is a type offlat panel display used in various electronic devices. At a minimum, aLCD comprises a substrate, at least one conductive layer and a liquidcrystal layer. LCDs may also comprise two sheets of polarizing materialwith a liquid crystal solution between the polarizing sheets. The sheetsof polarizing material may comprise a substrate of glass or transparentplastic. The LCD may also include functional layers. In one embodimentof a LCD, a transparent, multilayer flexible support is coated with afirst conductive layer, which may be patterned, onto which is coated thelight modulating liquid crystal layer. A second conductive layer isapplied and overcoated with a dielectric layer to which dielectricconductive row contacts are attached, including via holes that permitinterconnection between conductive layers and the dielectric conductiverow contacts. An optional nanopigmented functional layer may be appliedbetween the liquid crystal layer and the second conductive layer.

The liquid crystal (LC) is used as an optical switch. The substrates areusually manufactured with transparent, conductive electrodes, in whichelectrical “driving” signals are coupled. The driving signals induce anelectric field which can cause a phase change or state change in theliquid crystal material, thus exhibiting different light reflectingcharacteristics according to its phase and/or state.

Cholesteric liquid crystals are bistable at zero field and drive schemesmay be designed based on their response to voltage pulses. A typicalresponse of a bistable cholesteric or chiral nematic liquid crystalmaterial to voltage pulses is shown in FIG. 8. The horizontal axisrepresents the amplitude of the addressing voltage pulse and thevertical axis represents the reflectance measured after the liquidcrystal relaxes to the stable state following application of the voltagepulse. The solid line is the response, when the material is initially inthe planar state or texture, and the dashed line is the response, whenthe material is initially in the focal conic texture. In theconventional drive scheme for bistable cholesteric displays, thedisplays are addressed row by row. With reference to FIG. 8, if the rowvoltage VR is set at VR=(V3+V4)/2, then the column voltage VC has to bewithin the range V4−VR<VC<V1 for all columns in order to derive maximumcontrast without cross-talk in a matrix or multiplexed display. Bywithout cross-talk, it is meant that the portion of the image that hasalready been written on a multi-row display device will not be alteredwhen a new row is selected and addressed. From the above relationships,it follows that for maximum contrast without cross-talk, (V4−V3)/2<V1.If we define a quantity Vqm=2V1/(V4−V3), it is clearly desirable thatVqm is greater than or at least equal to 1.

The displays may employ any suitable driving schemes and electronicsknown to those skilled in the art, including the following, all of whichare incorporated herein by reference in their entireties: Doane, J. W.,Yang, D. K., Front—lit Flat Panel Display from Polymer StabilizedCholesteric Textures, Japan Display 92, Hiroshima October 1992; Yang, D.K. and Doane, J. W., Cholesteric Liquid Crystal/Polymer Gel Dispersion:Reflective Display Application, SID Technical Paper Digest, Vol XXIII,May 1992, p. 759, et sea.; U.S. patent application Ser. No. 08/390,068,filed Feb. 17, 1995, entitled “Dynamic Drive Method and Apparatus for aBistable Liquid Crystal Display” and U.S. Pat. No. 5,453,863.

A typical display in its simplest form comprises a sheet supporting aconventional polymer dispersed electrically modulated material. Thesheet includes a substrate. Substrate can be made of a polymericmaterial, such as Kodak Estar film base formed of polyester plastic, andhave a thickness of between 20 and 200 microns. For example, substratecan be a 80 micron thick sheet of transparent polyester. Other polymers,such as transparent polycarbonate, can also be used. Alternatively,substrate can be thin, transparent glass.

A first conductor is formed over substrate. First conductor can be atransparent, electrically conductive layer of tin-oxide orindium-tin-oxide (ITO), with ITO being the preferred material.Typically, first conductor is sputtered onto the substrate to aresistance of less than 250 ohms per square. Alternatively, firstconductor can be an opaque electrical conductor formed of metal such ascopper, aluminum or nickel. If first conductor is an opaque metal, themetal can be a metal oxide to create a light absorbing first conductor.A second conductor may be applied to the surface of light modulatingimaging layer. Second conductor should have sufficient conductivity tocarry a field across light modulating imaging layer. Second conductorcan be formed in a vacuum environment using materials such as aluminum,tin, silver, platinum, carbon, tungsten, molybdenum, or indium.

In a preferred embodiment of the invention, the display device ordisplay sheet has simply a single imaging layer of liquid crystalmaterial along a line perpendicular to the face of the display,preferably a single layer coated on a flexible substrate. Such asstructure, as compared to vertically stacked imaging layers each betweenopposing substrates, is especially advantageous for monochrome shelflabels and the like. Structures having stacked imaging layers, however,are optional for providing additional advantages in some case.

In a typical matrix-addressable light emitting display device, numerouslight emitting devices are formed on a single substrate and arranged ingroups in a regular grid pattern. Activation may be by rows and columns,or in an active matrix with individual cathode and anode paths. OLEDsare often manufactured by first depositing a transparent electrode onthe substrate, and patterning the same into electrode portions. Theorganic layer(s) is then deposited over the transparent electrode. Ametallic electrode can be formed over the electrode layers. For example,in U.S. Pat. No. 5,703,436 to Forrest et al., incorporated herein byreference, transparent indium tin oxide (ITO) is used as the Holeinjecting electrode, and a Mg——Ag-ITO electrode layer is used forelectron injection.

The flexible plastic substrate can be any flexible self-supportingplastic film that supports the thin conductive metallic film. “Plastic”means a high polymer, usually made from polymeric synthetic resins,which may be combined with other ingredients, such as curatives,fillers, reinforcing agents, colorants, and plasticizers. Plasticincludes thermoplastic materials and thermosetting materials.

The flexible plastic film must have sufficient thickness and mechanicalintegrity so as to be self-supporting, yet should not be so thick as tobe rigid. Typically, the flexible plastic substrate is the thickestlayer of the composite film in thickness. Consequently, the substratedetermines to a large extent the mechanical and thermal stability of thefully structured composite film.

Another significant characteristic of the flexible plastic substratematerial is its glass transition temperature (Tg). Tg is defined as theglass transition temperature at which plastic material will change fromthe glassy state to the rubbery state. It may comprise a range beforethe material may actually flow. Suitable materials for the flexibleplastic substrate include thermoplastics of a relatively low glasstransition temperature, for example up to 150° C., as well as materialsof a higher glass transition temperature, for example, above 150° C. Thechoice of material for the flexible plastic substrate would depend onfactors such as manufacturing process conditions, such as depositiontemperature, and annealing temperature, as well as post manufacturingconditions such as in a process line of a displays manufacturer. Certainof the plastic substrates discussed below can withstand higherprocessing temperatures of up to at least about 200° C., some up to300-350° C., without damage.

Typically, the flexible plastic substrate is polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polyethersulfone (PES),polycarbonate (PC), polysulfone, a phenolic resin, an epoxy resin,polyester, polyimide, polyetherester, polyetheramide, cellulose acetate,aliphatic polyurethanes, polyacrylonitrile, polytetrafluoroethylenes,polyvinylidene fluorides, poly(methyl (x-methacrylates), an aliphatic orcyclic polyolefin, polyarylate (PAR), polyetherimide (PEI),polyethersulphone (PES), polyimide (PI), Teflon poly(perfluoro-alboxy)fluoropolymer (PFA), poly(ether ether ketone) (PEEK), poly(ether ketone)(PEK), poly(ethylene tetrafluoroethylene) fluoropolymer (PETFE), andpoly(methyl methacrylate) and various acrylate/methacrylate copolymers(PMMA). Aliphatic polyolefins may include high density polyethylene(HDPE), low density polyethylene (LDPE), and polypropylene, includingoriented polypropylene (OPP). Cyclic polyolefins may includepoly(bis(cyclopentadiene)). A preferred flexible plastic substrate is acyclic polyolefin or a polyester. Various cyclic polyolefins aresuitable for the flexible plastic substrate. Examples include Arton®made by Japan Synthetic Rubber Co., Tokyo, Japan, Zeanor T made by ZeonChemicals L.P., Tokyo Japan, and Topas® made by Celanese A. G., KronbergGermany. Arton is a poly(bis(cyclopentadiene)) condensate that is a filmof a polymer. Alternatively, the flexible plastic substrate can be apolyester. A preferred polyester is an aromatic polyester such asArylite. Although various examples of plastic substrates are set forthabove, it should be appreciated that the substrate can also be formedfrom other materials such as glass and quartz.

The flexible plastic substrate can be reinforced with a hard coating.Typically, the hard coating is an acrylic coating. Such a hard coatingtypically has a thickness of from 1 to 15 microns, preferably from 2 to4 microns and can be provided by free radical polymerization, initiatedeither thermally or by ultraviolet radiation, of an appropriatepolymerizable material. Depending on the substrate, different hardcoatings can be used. When the substrate is polyester or Arton, aparticularly preferred hard coating is the coating known as “Lintec”.Lintec contains UV cured polyester acrylate and colloidal silica. Whendeposited on Arton, it has a surface composition of 35 atom % C, 45 atom% 0, and 20 atom % Si, excluding hydrogen. Another particularlypreferred hard coating is the acrylic coating sold under the trademark“Terrapin” by Tekra Corporation, New Berlin, Wis.

The LCD contains at least one conductive layer, which typically iscomprised of a primary metal oxide. This conductive layer may compriseother metal oxides such as indium oxide, titanium dioxide, cadmiumoxide, gallium indium oxide, niobium pentoxide and tin dioxide. See,Int. Publ. No. WO 99/36261 by Polaroid Corporation. In addition to theprimary oxide such as ITO, the at least one conductive layer can alsocomprise a secondary metal oxide such as an oxide of cerium, titanium,zirconium, hafnium and/or tantalum. See, U.S. Pat. No. 5,667,853 toFukuyoshi et al. (Toppan Printing Co.) Other transparent conductiveoxides include, but are not limited to ZnO₂, Zn₂SnO₄, Cd₂SnO₄, Zn₂In₂O₅,MgIn₂O₄, Ga₂O₃—In₂ 0 ₃, or TaO₃. The conductive layer may be formed, forexample, by a low temperature sputtering technique or by a directcurrent sputtering technique, such as DC sputtering or RF-DC sputtering,depending upon the material or materials of the underlying layer. Theconductive layer may be a transparent, electrically conductive layer oftin oxide or indium tin oxide (ITO), or polythiophene (PDOT). Typically,the conductive layer is sputtered onto the substrate to a resistance ofless than 250 ohms per square. Alternatively, conductive layer may be anopaque electrical conductor formed of metal such as copper, aluminum ornickel. If the conductive layer is an opaque metal, the metal can be ametal oxide to create a light absorbing conductive layer.

The material is coated over patterned ITO first conductors to provide apolymer dispersed cholesteric coating having a dried thickness of lessthan 50 microns, preferably less than 25 microns, more preferably lessthan 15 microns, most preferably less than about 10 microns.

Indium tin oxide (ITO) is the preferred conductive material, as it is acost effective conductor with good environmental stability, up to 90%transmission, and down to 20 ohms per square resistivity. An exemplarypreferred ITO layer has a % T greater than or equal to 80% in thevisible region of light, that is, from greater than 400 nm to 700 nm, sothat the film will be useful for display applications. In a preferredembodiment, the conductive layer comprises a layer of low temperatureITO, which is polycrystalline. The ITO layer is preferably 10-120 nm inthickness, or 50-100 nm thick to achieve a resistivity of 20-60ohms/square on plastic. An exemplary preferred ITO layer is 60-80 nmthick.

The conductive layer is preferably patterned. The conductive layer ispreferably patterned into a plurality of electrodes. The patternedelectrodes may be used to form a LCD device. In another embodiment, twoconductive substrates are positioned facing each other and cholestericliquid crystals are positioned there between to form a device. Thepatterned ITO conductive layer may have a variety of dimensions.Exemplary dimensions may include line widths of 10 microns, distancesbetween lines, that is, electrode widths, of 200 microns, depth of cut,that is, thickness of ITO conductor, of 100 nanometers. ITO thicknesseson the order of 60, 70, and greater than 100 nanometers are alsopossible.

The display may also contain a second conductive layer applied to thesurface of the light modulating layer. The second conductive layerdesirably has sufficient conductivity to carry a field across the lightmodulating layer. The second conductive layer may be formed in a vacuumenvironment using materials such as aluminum, tin, silver, platinum,carbon, tungsten, molybdenum, or indium. Oxides of these metals can beused to darken patternable conductive layers. The metal material can beexcited by energy from resistance heating, cathodic arc, electron beam,sputtering or magnetron excitation. The second conductive layer maycomprise coatings of tin oxide or indium tin oxide, resulting in thelayer being transparent. Alternatively, second conductive layer may beprinted conductive ink.

For higher conductivities, the second conductive layer may comprise asilver based layer which contains silver only or silver containing adifferent element such as aluminum (Al), copper (Cu), nickel (Ni),cadmium (Cd), gold (Au), zinc (Zn), magnesium (Mg), tin (Sn), indium(In), tantalum (Ta), titanium (Ti), zirconium (Zr), cerium (Ce), silicon(Si), lead (Pb) or palladium (Pd). In a preferred embodiment, theconductive layer comprises at least one of gold, silver and agold/silver alloy, for example, a layer of silver coated on one or bothsides with a thinner layer of gold. See, Int. Publ. No. WO 99/36261 byPolaroid Corporation. In another embodiment, the conductive layer maycomprise a layer of silver alloy, for example, a layer of silver coatedon one or both sides with a layer of indium cerium oxide (InCeO). SeeU.S. Pat. No. 5,667,853, incorporated herein in by reference.

The second conductive layer may be patterned irradiating themultilayered conductor/substrate structure with ultraviolet radiation sothat portions of the conductive layer are ablated therefrom. It is alsoknown to employ an infrared (IR) fiber laser for patterning a metallicconductive layer overlying a plastic film, directly ablating theconductive layer by scanning a pattern over the conductor/filmstructure. See: Int. Publ. No. WO 99/36261 and “42.2: A New ConductorStructure for Plastic LCD Applications Utilizing ‘All Dry’ Digital LaserPatterning,” 1998 SID International Symposium Digest of TechnicalPapers, Anaheim, Calif., May 17-22, 1998, no. VOL. 29, May 17, 1998,pages 1099-1101, both incorporated herein by reference.

In a preferred embodiment, second conductors are printed conductive inksuch as ELECTRODAG 423SS screen printable electrical conductive materialfrom Acheson Corporation. Such printed materials are finely dividedgraphite particles in a thermoplastic resin. The second conductors areformed using printed inks to reduce display cost. The use of a flexiblesupport for substrate layer, laser etched first conductors, machinecoating polymer dispersed cholesteric layer, and printing secondconductors permit the fabrication of very low cost memory displays.Small displays formed using these methods can be used as electronicallyrewritable transaction cards for inexpensive, limited rewriteapplications.

A light absorbing second conductor may be positioned on the sideopposing the incident light. In the fully evolved focal conic state thecholesteric liquid crystal is transparent, passing incident light, whichis absorbed by second conductor to provide a black image. Progressiveevolution to the focal conic state causes a viewer to see an initialbright reflected light that transitions to black as the cholestericmaterial changes from planar state to a fully evolved focal conic state.The transition to the light transmitting state is progressive, andvarying the low voltage time permits variable levels of reflection.These variable levels can be mapped out to corresponding gray levels,and when the field is removed, light modulating layer maintains a givenoptical state indefinitely. The states are more fully discussed in U.S.Pat. No. 5,437,811.

In addition to a second conductive layer, other means may be used toproduce a field capable of switching the state of the liquid crystallayer as described in, for example, U.S. Pat Appl. Nos. 20010008582 A1,20030227441 A1, 20010006389 A1, and U.S. Pat. Nos. 6,424,387, 6,269,225,and 6,104,448, all incorporated herein by reference.

The LCD may also comprise at least one “functional layer” between theconductive layer and the substrate. The functional layer may comprise aprotective layer or a barrier layer. The protective layer useful in thepractice of the invention can be applied in any of a number of wellknown techniques, such as dip coating, rod coating, blade coating, airknife coating, gravure coating and reverse roll coating, extrusioncoating, slide coating, curtain coating, and the like. The liquidcrystal particles and the binder are preferably mixed together in aliquid medium to form a coating composition. The liquid medium may be amedium such as water or other aqueous solutions in which the hydrophiliccolloid are dispersed with or without the presence of surfactants. Apreferred barrier layer may acts as a gas barrier or a moisture barrierand may comprise SiOx, AlOx or ITO. The protective layer, for example,an acrylic hard coat, functions to prevent laser light from penetratingto functional layers between the protective layer and the substrate,thereby protecting both the barrier layer and the substrate. Thefunctional layer may also serve as an adhesion promoter of theconductive layer to the substrate.

In another embodiment, the polymeric support may further comprise anantistatic layer to manage unwanted charge build up on the sheet or webduring roll conveyance or sheet finishing. In another embodiment of thisinvention, the antistatic layer has a surface resistivity of between 10⁵to 10¹² ohms. Above 10¹², the antistatic layer typically does notprovide sufficient conduction of charge to prevent charge accumulationto the point of preventing fog in photographic systems or from unwantedpoint switching in liquid crystal displays. While layers greater than10⁵ will prevent charge buildup, most antistatic materials areinherently not that conductive and in those materials that are moreconductive than 10⁵, there is usually some color associated with themthat will reduce the overall transmission properties of the display. Theantistatic layer is separate from the highly conductive layer of ITO andprovides the best static control when it is on the opposite side of theweb substrate from that of the ITO layer. This may include the websubstrate itself.

Another type of functional layer may be a color contrast layer. Colorcontrast layers may be radiation reflective layers or radiationabsorbing layers. In some cases, the rearmost substrate of each displaymay preferably be painted black. The color contrast layer may also beother colors. In another embodiment, the dark layer comprises millednonconductive pigments. The materials are milled below 1 micron to form“nanopigments”. In a preferred embodiment, the dark layer absorbs allwavelengths of light across the visible light spectrum, that is from 400nanometers to 700 nanometers wavelength. The dark layer may also containa set or multiple pigment dispersions. Suitable pigments used in thecolor contrast layer may be any colored materials, which are practicallyinsoluble in the medium in which they are incorporated. Suitablepigments include those described in Industrial Organic Pigments:Production, Properties, Applications by W. Herbst and K. Hunger, 1993,Wiley Publishers. These include, but are not limited to, Azo Pigmentssuch as monoazo yellow and orange, diazo, naphthol, naphthol reds, azolakes, benzimidazolone, diazo condensation, metal complex, isoindolinoneand isoindolinic, polycyclic pigments such as phthalocyanine,quinacridone, perylene, perinone, diketopyrrolo-pyrrole, and thioindigo,and anthriquinone pigments such as anthrapyrimidine.

The functional layer may also comprise a dielectric material. Adielectric layer, for purposes of the present invention, is a layer thatis not conductive or blocks the flow of electricity. This dielectricmaterial may include a UV curable, thermoplastic, screen printablematerial, such as Electrodag 25208 dielectric coating from AchesonCorporation. The dielectric material forms a dielectric layer. Thislayer may include openings to define image areas, which are coincidentwith the openings. Since the image is viewed through a transparentsubstrate, the indicia are mirror imaged. The dielectric material mayform an adhesive layer to subsequently bond a second electrode to thelight modulating layer.

The liquid crystal containing composition of the invention can beapplied by any of a number of well known techniques, such as dipcoating, rod coating, blade coating, air knife coating, slide (or bead)coating, curtain coating, and the like.

After coating, the layer is generally dried by simple evaporation, whichmay be accelerated by known techniques such as convection heating. Knowncoating and drying methods are described in further detail in ResearchDisclosure No. 308119, Published December 1989, pages 1007 to 1008. Thecoating is maintained above the chill set temperature or sol-geltransition temperature of gelatin during drying of the imaging layer orlight modulating layer to permit self-assembly of the droplets into aclose-packed monolayer.

A coated sheet can be formed using inexpensive, efficient layeringmethods. A single large volume of sheet material can be coated andformed into various types of smaller sheets for use in display devicessuch as transaction cards, shelf labels, large format signage, and thelike. Displays in the form of sheets in accordance with the presentinvention are inexpensive, simple, and fabricated using low costprocesses.

In the preferred embodiment, the imaging layer or light modulating layeris first applied and maintained above the chill set temperature orsol-gel transition temperature of gelatin. After drying, the binder inthis layer is allowed to crosslink to preserve the close-packedmonolayer distribution of coated droplets. A second aqueous layercontaining gelatin is then applied. The coating is chill set prior todrying of the second layer in order to allow the gelatin molecules inthe second layer to adopt a helix structure In a preferred commercialembodiment, the substrate to be coated is in the form of a moving web.After completing the manufacture of a coated liquid crystal sheetmaterial between spaced electrodes, the sheet material can be cut into aplurality of smaller, individual areas for use in various display orother applications.

A close-packed monolayer of uniform thickness may provide enhancedperformance with respect to surface roughness. In conventional liquidcrystal coatings containing non-uniform droplets or capsules, the rootmean square (RMS) surface roughness has been measured at about 6microns. This is a very high value of surface roughness that wouldresult in irregular or incomplete curing if a UV curable screen printedconducive ink is used as the second electrode. The irregular curing willresult in increased switching voltages. Furthermore, a surface roughnessof this magnitude will also result in significant non-uniformity ofswitching voltage across the area of the display since the switchingvoltage is directly related to the thickness of the coated layer. Theself-assembled droplets or domains in the present close-packed monolayerdemonstrates a RMS surface roughness of less than 1.5 microns, morepreferably less than 1.0 microns and most preferably less than 0.5microns.

The following examples are provided to illustrate the invention.

EXAMPLE 1

This example illustrates the close-packed ordered monolayer structureobtained according to the invention compared to the random structure ofa control coating.

Chiral nematic liquid crystal (CLC) compositions with center wavelengthsof reflection (CWR) at 540 nm and 590 nm were prepared by adding theappropriate amount of a high twist chiral dopant to the nematic hostmixture BL087 obtained from Merck, Darmstadt, Germany. The CLCcomposition with CWR at 590 nm also contained 0.2% weight/weight (w/w)of the blue absorbing dye Neopen Yellow 057 from BASF Corporation.

Method I (Invention)

A dispersion of the CLC composition with CWR at 590 nm was prepared asfollows. To 241 grams of distilled water was added 3.6 grams of Ludox TMcolloidal silica suspension and 7.4 grams of a 10% w/w aqueous solutionof a copolymer of methylaminoethanol and adipic acid. To this was added108 grams of the CLC composition. The mixture was stirred using aSilverson mixer at 5000 rpm. It was then passed through a microfluidizerat 3000 psi. Finally, the resulting dispersion was passed through a 23μm filter. The droplet size distribution in the dispersion was measuredusing a Coulter Counter. It was found that the mean size was 9.7 micronswith a coefficient of variation (cv) of 0.14.

The above dispersion was mixed with an aqueous solution of fish skingelatin from Norland Products Inc. having a weight-average molecularweight of 83,800 and a polydispersity of 3.4, an aqueous solution ofpolyvinylalcohol (PVA)(type GL-05 from Nippon Gohsei Limited), asolution of Aerosol OT in water and a solution ofbis(vinylsulfonyl)methane in water to give a coating compositioncontaining 15% w/w CLC material, 4.5% fish skin gelatin, 0.5% PVA, 0.07%Aerosol OT coating aid and 0.1% bis(vinylsulfonyl)methane crosslinker.The composition was spread over a plastic support with a thin layer ofindium tin oxide (ITO) at 37.67 cm³/m² to give a dry uniform coverage ofabout 5400 mg/m² of CLC material. The PVA prevented agglomeration of theliquid crystal domains during self-assembly and helped minimize defectsin the coating. The plastic support (Dupont ST504) with a sputter coatedITO conductive layer (300 ohm/sq resistivity) was obtained from Bekaert.The thickness of the ITO layer is approximately 240 Angstroms. Duringthe operation, the plastic support was placed over a coating block thatwas maintained at room temperature (23° C.) and the coating compositionwas also delivered or applied at the same temperature. The resultingcoating was then dried under ambient conditions (23° C.). The fishgelatin does not chill set at room temperature and, thus, had not chillset at the onset of drying.

A sample of the dried coating was positioned facing a helium-neon laser(Spectra Physics model 155A). A screen was placed behind the sample. Thedistance from the source of laser light to the sample was 20.6 cm andthe distance from the sample to the screen was 10.5 cm. As shown in FIG.1, the invention coating showed diffraction rings on the screen causedby Fraunhofer diffraction of light indicating an ordered close-packedmonolayer of CLC droplets in the coating.

Method 2 (Control)

A dispersion of the CLC composition with CWR at 540 nm was prepared asfollows. To 172.1 grams of distilled water was added 2.6 grams of LudoxTM colloidal silica suspension and 5.3 grams of a 10% w/w aqueoussolution of a copolymer of methylaminoethanol and adipic acid. To thiswas added 77.2 grams of the CLC composition. The mixture was stirredusing a Silverson mixer at 5000 rpm. It was then passed through amicrofluidizer at 3000 psi. Finally, the resulting dispersion was passedthrough a 23 μm filter. The droplet size distribution in the dispersionwas measured using a Coulter Counter. It was found that the mean sizewas 9.4 microns with a coefficient of variation (cv) of 0.14.

The above dispersion was mixed with an aqueous solution of conventionalType IV cattle gelatin having a weight-average molecular weight of229,000 and a polydispersity of 5.4, a solution of Alkanol XC in waterand a solution bis(vinylsulfonyl)methane in water to give a coatingcomposition containing 8% w/w CLC material, 5% gelatin, 1% Alkanol XCcoating aid and 0.05% bis(vinylsulfonyl)methane crosslinker. Thecomposition was then spread over a plastic support with a thin layer ofindium tin oxide (ITO) to give a uniform coverage of about 5400 mg/m².The plastic support (Dupont ST504) with a sputter coated ITO conductivelayer (300 ohm/sq resistivity) was obtained from Bekaert.. The thicknessof the ITO layer is approximately 240 Angstroms. During the operation,the coating block was maintained at a temperature close to roomtemperature (23° C.) and the coating composition was delivered orapplied at a temperature of 45° C. The resulting coating was then driedunder ambient conditions (23° C.). It should be noted that conventionalgelatin will chill set at this temperature prior to the onset of dryingsince the sol-gel transition temperature of conventional gelatin isabove room temperature.

A sample of this coating was inspected using the same laser light sourceunder the same conditions as before. As shown in FIG. 2, in this casethe diffraction pattern is distinctly different, indicative of a verydisordered or random coating of the CLC droplets.

EXAMPLE 2

This example illustrates the improved electro-optical properties of adevice fabricated according to the invention compared to a controldevice.

Method I (Invention)

A dispersion of CLC material was prepared in a manner similar to MethodI of Example 1.

The dispersion was mixed with an aqueous solution of Type IV cattlegelatin having a weight-average molecular weight of 117,000 and apolydispersity of 3.7, an aqueous solution of polyvinylalcohol(PVA)(type GL-05 from Nippon Gohsei Limited), a solution of Aerosol OTin water and a solution bis(vinylsulfonyl)methane in water to give acoating composition containing 15% w/w CLC material, 4.5% gelatin, 0.5%PVA, 0.07% Aerosol OT and 0.1% bis(vinylsulfonyl)methane. Thecomposition was spread over a plastic support with a thin layer ofindium tin oxide (ITO) to give a uniform coverage of about 5400 mg/M² ofCLC material. The coated layer was referred to as layer 1. During theoperation, the plastic support was placed over a coating block that wasmaintained at a temperature of 45° C. and the coating composition wasalso delivered or applied at the same temperature.

The resulting coating was then dried at 45° C. The coating was keptaside for 48 hours to allow the crosslinking of gelatin to go tocompletion. Analysis of a portion of the coating by laser light as inExample 1 showed rings due to Fraunhofer diffraction of light. Analysisof a second portion of the coating by x-ray diffraction showed acomplete absence of triple helix structure in gelatin. In other words,the gelatin molecules in layer 1 were in a random coil configuration.The coating was then placed on a coating block that was maintained at23° C. A 4% w/w solution of Type IV cattle gelatin was then coated overlayer 1 at a uniform wet coverage of about 28 cm³/m². This second layerwas referred to as layer 2. The coating was then dried at roomtemperature (23° C.).

Analysis of a portion of the coating by x-ray diffraction now showed apeak at low angle associated with the triple helix structure of gelatinindicating that at least some of the gelatin molecules in layer 2 werein the triple helix configuration. Analysis by laser light showed thatthe close-packed structure had been preserved even after the secondlayer had been applied. A carbon-based conductive ink (Electrodag 423SSfrom Acheson Corporation) was then screen printed on top of layer 2 tocomplete the construction of the display device.

The electro-optic response of the device is shown in FIG. 3. Thehorizontal axis is the amplitude of the addressing voltage pulse and thevertical axis is the reflectance of the CLC material measured at 0 voltsabout 2 seconds after application of the addressing pulse. Thereflectance was measured using an X-rite 938 spectrodensitometer. Theaddressing pulse was a square wave with frequency of 250 Hz and durationof 100 milliseconds. The open circles represent the response when thematerial was initially in the planar texture and the filled circlesrepresent the response when the material was initially in the focalconic state. A voltage pulse higher than 70 volts (V4) switched thedisplay into the reflecting state and a voltage pulse between 44 and 54volts (V2 and V3 respectively) switched the display into the weaklyscattering or dark state. Voltages less than V1 (8 volts) did notinfluence the state of the display. In this case the quantity Vqm,defined as 2V1/(V4−V3), was equal to 1. For a multi-row and columnmatrix display using conventional drive it is important that Vqm isgreater than or equal to 1 to ensure maximum contrast withoutcross-talk.

Method 2 (Control)

A dispersion of CLC material and coating composition were prepared in amanner similar to Method 2 of Example 1. The composition was coated asdescribed under Method 2 of Example 1. A carbon-based conductive ink(Electrodag 423SS from Acheson Corporation) was then screen printed ontop of the coating to create the display device. The electro-opticresponse of the device is shown in FIG. 4. In this case, a voltage pulsehigher than 130 volts (V4) switched the display into the reflecting(bright) state and a voltage pulse between 44 and 64 volts (V2 and V3respectively) switched the display into the weakly scattering or darkstate. The contrast ratio defined as the reflectance of the bright statedivided by that of the dark state was 15.8. Voltages less than 14 volts(V1) did not influence the state of the display. Based on theseparameters Vqm was only 0.42.

It is clear that the voltage needed to switch the display into thereflecting state was significantly greater than that of the inventiondevice. Furthermore, the contrast of a multiplexed display based on thismethod may be greatly inferior compared to the method of the inventionbecause of the relatively low value of Vqm.

EXAMPLE 3 (Invention)

A dispersion of CLC material and coating composition were prepared asdescribed under Method 1 of Example 1. The composition was spread over aplastic support (Dupont ST504) with a thin layer of ITO (approximately240 Angstroms in thickness and 300 ohm/sq resistivity) at 37.7 cm³/m² togive a uniform dry coverage of about 5400 mg/m² for the CLC material.During the operation, the plastic support was placed over a coatingblock that was maintained at 45 C and the coating composition wasdelivered at 23 C. The resulting coating of the imaging layer was driedat 45 C. All of these temperatures were above the sol-gel transitiontemperature of the binder.

The coating of the imaging layer was kept aside for 48 h to allow thecross-linking of gelatin to go to completion. The coating was thenplaced on a coating block that was maintained at 30 C. A compositioncontaining 4% Type IV cattle gelatin, 1.4% carbon black and 0.1% AerosolOT in distilled water was then spread over it using a coating knife with0.008 cm gap to constitute the protective overcoat. The coating wasdried at 30 C. For this top layer or protective overcoat, thetemperature of the coating block and the drying temperature were bothbelow the sol-gel transition temperature of the binder. A silver-basedconducting ink was then screen printed over the dried protective layerto complete construction of the display device. The electro-opticresponse of this device is shown in FIG. 5. A voltage pulse higher than72 volts (V4) switched the display into the reflecting state and avoltage pulse between 32 and 44 volts (V2 and V3 respectively) switchedthe display into the weakly scattering or dark state. The contrast ratiowas 33.1.

It is clear that the voltages needed to switch the display into thebright state and the dark state were considerably lower than those forthe control.

Furthermore, the contrast ratio was significantly higher indicating amuch improved display at low voltage.

EXAMPLE 4 (Invention)

This example illustrates the uniformity of thickness of the imaginglayer using the method of the invention. It is preferred that the RMSsurface roughness is less than 1.5 microns, it is more preferred thatthe RMS surface roughness is less than 1.0 microns and it is mostpreferred that the RMS surface roughness is less than 0.5microns.(reasons for the uniformity of thickness are in the prior artsection).

A dispersion of CLC material and coating composition were prepared asdescribed under Method 1 of Example 1. The composition was spread over aplastic support (Dupont ST504) with a thin layer of ITO (approximately240 Angstroms in thickness and 300 ohm/sq resistivity) at 37.7 cm³/m² togive a uniform dry coverage of about 5400 mg/m² for the CLC material.During the operation, the plastic support was placed over a coatingblock that was maintained at 45 C and the coating composition wasdelivered at 23 C. The resulting coating of the imaging layer was driedat 45 C. All of these temperatures were above the sol-gel transitiontemperature of the binder.

The coating of the imaging layer was kept aside for 48 hours to allowthe crosslinking of gelatin to go to completion. The surface roughnessprofile was then obtained using a Zeiss LSM 510 laser scanningmicroscope from Carl Zeiss Microimaging Inc., One Zeiss Drive, ThornwoodN.Y. 10594. FIG. 9 shows a micrograph of the coating of the imaginglayer along with the surface roughness profile. The roughness profileshows the variation in thickness as a function of distance along a linedrawn across the surface of the coating. From such a profile it ispossible to calculate the root mean square (RMS) surface roughness (seefor example Wyko NT8000 Set Up and Operation Guide June 2003 version3.01 Veeco Industries Inc. 2650 East Elvira Road, Tuscon Ariz. 85706).For this coating the RMS surface roughness was about 0.4 microns.

The coating was then immersed in acetone to remove the liquid crystalmaterial. After removal of the liquid crystal material, the coating wasonce again examined using the laser confocal microscope. Thecorresponding micrograph is shown in FIG. 10. The coating now showswell-defined cavities in areas that were originally occupied by dropletsof the liquid crystal. Note that the crosslinked binder resisteddissolution. In other words, cross-linking the binder allowed theclose-packed structure to be fixed or locked into place. The attachedprofile shows that the imaging layer had a uniform thickness of about 5microns.

EXAMPLE 5 (Control)

This example illustrates the effect of using a polymer latex as thebinder material in the imaging layer instead of gelatin.

A CLC composition with CWR at 450 nm was prepared by adding theappropriate amount of a high twist chiral dopant to the nematic hostmixture BL087 obtained from Merck, Darmstadt, Germany.

A dispersion of the CLC composition with CWR at 450 nm was prepared asfollows. To 166 grams of distilled water was added 2.6 grams of Ludox TMcolloidal silica suspension and 5.1 grams of a 10% w/w aqueous solutionof a copolymer of methylaminoethanol and adipic acid. To this was added74.6 grams of the CLC composition. The mixture was stirred using aSilverson mixer at 5000 rpm. It was then passed through a microfluidizerat 3000 psi. Finally, the resulting dispersion was passed through a 23μm filter. The droplet size distribution in the dispersion was measuredusing a Coulter Counter. It was found that the mean size was 9.4 micronswith a coefficient of variation (cv) of 0.14.

The above dispersion was mixed with an aqueous suspension ofpolyurethane polymer latex NeoRez R-967 from NeoResins, WilmingtonMass., USA and a solution of coating aid Olin 10 G in water to give acoating composition containing 15% w/w CLC material, 5.0% polymer latexand 0.1% Olin 10 G. The composition was spread over a plastic supportwith a thin layer of indium tin oxide (ITO) using a coating knife with0.008 cm gap. The plastic support (Dupont ST504) with a sputter coatedITO conductive layer (300 ohm/sq resistivity) was obtained from Bekaert.The thickness of the ITO layer is approximately 240 Angstroms. Duringthe operation, the plastic support was placed over a coating block thatwas maintained at room temperature (23° C.) and the coating compositionwas also delivered or applied at the same temperature. The resultingcoating was then dried under ambient conditions (23° C.). The coatingwas then placed on a coating block that was maintained at 23 C. Acomposition containing 4% Type IV cattle gelatin and 0.1% Aerosol OT indistilled water was then spread over it using a coating knife with 0.008cm gap to constitute the protective overcoat. The coating was dried at23 C. For this top layer or protective overcoat, the temperature of thecoating block and the drying temperature were both below the sol-geltransition temperature of the binder. A carbon-based conductive ink(Electrodag 423SS from Acheson Corporation) was then screen printed overthe protective overcoat to complete the construction of the displaydevice.

The electro-optic response of the device is shown in FIG. 11. Thehorizontal axis is the amplitude of the addressing voltage pulse and thevertical axis is the reflectance of the CLC material measured at 0 voltsabout 2 seconds after application of the addressing pulse. Thereflectance was measured using an X-rite 938 spectrodensitometer. Theaddressing pulse was a square wave with frequency of 250 Hz and durationof 100 milliseconds. The open circles represent the response when thematerial was initially in the planar texture and the filled circlesrepresent the response when the material was initially in the focalconic state. A voltage pulse higher than 138 volts (V4) switched thedisplay into the reflecting state and a voltage pulse between 64 and 86volts (V2 and V3 respectively) switched the display into the weaklyscattering or dark state. Voltages less than V1 (12 volts) did notinfluence the state of the display. In this case the quantity Vqm,defined as 2V1/(V4−V3), was equal to 0.46. Once again it is clear thatthe voltage needed to switch the display into the reflecting state wassignificantly greater than that of the invention device. Furthermore,the contrast of a multiplexed display based on this method may begreatly inferior compared to the method of the invention because of therelatively low value of Vqm.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A high contrast reflective display comprising at least one substrate,at least one electrically conductive layer and at least oneelectronically modulated imaging layer, wherein said electronicallymodulated imaging layer comprises a close-packed, ordered monolayer ofdomains of electrically modulated material in a fixed polymer matrix. 2.The high contrast reflective display of claim 1 wherein said substrateis flexible.
 3. The high contrast reflective display of claim 1 whereinsaid substrate is transparent.
 4. The high contrast reflective displayof claim 3 further comprising a second substrate.
 5. The high contrastreflective display of claim 1 wherein said electrically conductive layera surface conductivity of less than 10⁴ ohms/sq.
 6. The high contrastreflective display of claim 1 wherein said electrically conductiveconductive layer comprises ITO.
 7. The display of claim 1 wherein saidelectrically conductive layer comprises polythiophene.
 8. The highcontrast reflective display of claim 1 further comprising at least asecond electrically conductive layer, wherein said at least oneclose-packed, ordered monolayer of electrically modulated material in acrosslinked polymer matrix is between said at least one electricallyconductive layer and said second conductive layer.
 9. The high contrastreflective display of claim 1 wherein at least one electricallyconductive layer is patterned.
 10. The high contrast reflective displayof claim 1 further comprising a means for applying electrical fieldacross said at least one close-packed, ordered monolayer of electricallymodulated material in a fixed polymer matrix.
 11. The high contrastreflective display of claim 1 wherein said electrically modulatingmaterial is a liquid crystalline material.
 12. The high contrastreflective display of claim 1 wherein said electrically modulatedmaterial comprises a bistable liquid crystalline material.
 13. The highcontrast reflective display of claim 12 wherein said bistable liquidcrystalline material comprises a chiral nematic liquid crystal layer.14. The high contrast reflective display of claim 1 wherein said atleast one close-packed, ordered monolayer of electrically modulatedmaterial is a hexagonally closest packed monolayer of light modulatingmaterial.
 15. The high contrast reflective display of claim 1 whereinsaid at least one close-packed, ordered monolayer of electricallymodulated material has a repeat pattern or periodicity with a repeatdistance of the order of the wavelength of visible light.
 16. The highcontrast reflective display of claim 1 wherein said fixed polymer matrixis a crosslinked polymer matrix.
 17. The high contrast reflectivedisplay of claim 1 wherein said fixed polymer matrix is not rigid priorto drying.
 18. The high contrast reflective display of claim 1 whereinsaid fixed polymer matrix is in a random configuration if the coating isdried at a temperature that is above the sol-gel transition temperature.19. The high contrast reflective display of claim 1 wherein said fixedpolymer matrix prior to crosslinking has a sol-gel transitiontemperature or thermal gelation temperature or chill set temperaturebelow 23° C.
 20. The high contrast reflective display of claim 1 whereinsaid fixed polymer matrix prior to crosslinking has a sol-gel transitiontemperature or thermal gelation temperature or chill set temperaturebelow 10° C.
 21. The high contrast reflective display of claim 1 whereinsaid electrically modulated material to said fixed polymer matrix ratiois from 6:1 to 0.5:1
 22. The high contrast reflective display of claim 1wherein said electrically modulated material to said fixed polymermatrix ratio is 3:1.
 23. The high contrast reflective display of claim 1wherein said fixed polymer matrix comprises gelatin.
 24. The highcontrast reflective display of claim 23 wherein said gelatin is in arandom coil configuration if the coating is dried at a temperature thatis above the sol-gel transition temperature of gelatin.
 25. The highcontrast reflective display of claim 23 wherein said gelatin is fishgelatin.
 26. The high contrast reflective display of claim 25 whereinsaid fish gelatin is deep water fish gelatin.
 27. The high contrastreflective display of claim 23 wherein said gelatin has lower imino acidcontent than bovine gelatin.
 28. The high contrast reflective display ofclaim 1 wherein said fixed polymer matrix further comprises a hardeningagent.
 29. The high contrast reflective display of claim 1 furthercomprising a protective layer.
 30. The high contrast reflective displayof claim 29 wherein said protective layer comprises gelatin.
 31. Thehigh contrast reflective display of claim 30 wherein said gelatincontains molecules in a helix configuration.
 32. The high contrastreflective display of claim 1 wherein said electrically modulatedimaging layer further comprises polyvinylalcohol (PVA).
 33. The highcontrast reflective display of claim 1 wherein said at least oneelectronically modulated imaging layer has a RMS surface roughness ofless than 1.5 microns
 34. The high contrast reflective display of claim1 comprising a stack of at least two of said high contrast reflectivedisplays, wherein a stack comprises at least one substrate, at least oneelectrically conductive layer and at least one close-packed, orderedmonolayer of domains of electrically modulated material in a fixedpolymer matrix.
 35. A method of making a high contrast reflectivedisplay comprising: providing a substrate; applying a conductive layer;applying at least one layer of domains of electrically modulatedmaterial in a polymer matrix; drying the at least one applied layer ofsaid domains of electrically modulated material in a polymer matrix at atemperature above the chill set temperature or sol-gel transitiontemperature of said polymer matrix to permit self-assembly of saiddomains of electrically modulated material into a close-packed monolayerof said domains of electrically modulated material; and fixing saidpolymer matrix to preserve said close-packed monolayer of domains ofelectrically modulated material.
 36. The method of claim 35 furthercomprising: applying a second aqueous layer containing gelatin; chillsetting said second aqueous layer containing gelatin; and drying saidsecond aqueous layer.